1. Field of the Invention
This invention relates to a catalyst for the production of ethylene oxide which is used to produce ethylene oxide by the vapor oxidation of ethylene, and to a process for producing the catalyst. More particularly, this invention relates to a catalyst for the production of ethylene oxide in which silver, tungsten, cesium and, in some cases, other additive metals are deposited on a porous carrier.
2. Description of the Prior Art
The catalyst used for producing ethylene oxide on an industrial scale by the vapor contact oxidation of ethylene with molecular oxygen is a silver catalyst. In order to efficiently produce ethylene oxide, there is a strong demand for improved catalysts with highly selective and long-life catalysts being desired. Therefore, while various methods have been proposed in the past, these methods are primarily concerned with a combination of silver as a main active component with alkaline metal etc. as a reaction accelerator, optimization of the mixture ratio, and an improvement in carriers on which those components are to be deposited.
The specifications of GB 1473251 and GB 1512625 describe that high selectivity is obtained by a catalyst in which potassium, rubidium and/or cesium in a specific amount is deposited along with silver on a porous carrier. The specification of GB 1581884 describes that a catalyst containing silver and sodium, potassium, rubidium or cesium in specific amounts can improve the degree of activity and selectivity.
Also, the specification of U.S. Pat. No. 4,690,913 describes that silver grains deposited on a carrier are tiny and uniform and hence are highly active when used as a catalyst, and discloses a catalyst for the production of ethylene oxide in which the loading of silver grains is uniform from the outside surface layer to the inner layer of the catalyst.
On the other hand, when tungsten is used as a reaction accelerator, Japanese Patent Publication No. 61-21701, for example, discloses a catalyst in which silver and an alkaline metal tungstate are deposited on an .alpha.-alumina carrier containing a small amount of sodium. In this Publication, however, the amount of tungsten used is limited to the range of high concentration and the catalyst is defined as being effective only when the content of sodium in the carrier is not more than 0.07% by weight. Further, there is no suggestion found as to whether tungsten exhibits a remarkable accelerating effect or not when the amount of tungsten used is small.
The specification of EP 266015-A discloses a catalyst composition containing rhenium and tungsten each in an amount effective for acceleration, as well as silver. This specification describes that tungsten exhibits an accelerating effect in combination with rhenium, but nothing about the accelerating action of tungsten in the absence of rhenium. Further, the specification of EP 357292-A discloses a silver catalyst containing tungsten, an alkaline metal and sulfur. In this specification, however, the addition of sulfur is essential as a component of the catalyst, and it is not apparent whether a catalyst of high performance can be obtained or not in the absence of sulfur.